1. Field of the Invention
The present invention relates to systems and apparatus for heat transport. More specifically, the present invention relates to cooling systems and similar apparatus at cryogenic temperatures.
2. Description of the Related Art
Cryogenic coolers lift heat from infrared detectors and associated electronic components in applications where space is limited. The cryogenic cooler is typically inserted into a dewar (or housing) onto which one or more detector elements are mounted. Current missile applications require that an infrared focal plane array of detector elements be cooled to liquid nitrogen temperatures, i.e., 80.degree. K. (-193.degree. C.). Joule-Thomson and Stirling Cycle coolers are the two cooling technologies most often used to provide controlled cooling at such extreme temperatures.
A Joule-Thomson cryostat is a cooling device that uses a valve (known in the art as a "Joule-Thomson valve") through which a high pressure gas is allowed to expand via an irreversible throttling process in which enthalpy is conserved, resulting in lowering of its temperature. The simplest form of a conventional Joule-Thomson cryostat typically used a fixed-size orifice in the heat exchanger at the cold end of the cryostat such that cooling by the cryostat was unregulated. The input pressure and internal gas flow dynamics established the flow parameters of the coolant through the cryostat.
Joule-Thomson cryostats require a supply of highly pressurized gas. In certain applications, such as airborne missiles, this requirement tends to impose significant logistical constraints. In the airborne missile context, for example, only a limited amount of pressurized gas can be carried on the missile and the host vehicle. This limits the `on-station` life of the weapons platform in proportion to the amount of pressurized gas onboard. After each mission, the gas containers must be recharged. Frequent recharging of Joule-Thomson cryostats increases the possibility of contamination due to small impurities which may block the fine orifice of the device. These constraints impose significant field maintenance requirements which limit the viability of Joule-Thomson cryostats for such demanding applications.
For these and other reasons, Stirling cycle coolers tend to be preferred for such applications. A Stirling cycle cooler is an efficient and compact closed-cycle, electrically-driven cryogenic cooling device. The original Stirling cycle cooler consisted of a compressor piston within a cylinder, an expansion piston (or displacer) within a cylinder, and a drive mechanism. The drive mechanism converted the rotary motion of a motor and crankshaft to a reciprocating motion of the two pistons. The two pistons were arranged to be ninety degrees out-of-phase. A regenerative heat exchanger (regenerator) was included in the expansion piston to thermally isolate gas at the compressor piston head space from gas at the expansion piston head space. The original Stirling cycle cooler in which the compressor piston and expander piston are mechanically linked is known as the Integral Stirling Cooler. When operated between two temperature sinks, the Stirling cycle mechanism can either produce shaft power when heat is supplied to the expansion space (Stirling engine) or pump heat from a low temperature to a high temperature, and thereby provide refrigeration, when mechanical power is provided to the drive mechanism (Stirling cooler).
The Split-Stirling cycle cooler included all of the components of the integral Stirling cycle cooler, without mechanical linkage to the expander piston. This permitted the expander to be located remote from the compressor. The expander piston in this device was no longer driven by a connecting rod and crankshaft, but rather pneumatically by means of an additional drive piston. The drive piston was attached to the warm end of the expander piston and protruded into a small cavity at the extreme end of the expander housing. This created a "spring volume" as the gas acted as a spring on the drive piston.
The piston was sealed so that gas could not readily enter the spring volume from the expander side. The drive piston was pneumatically reciprocated by cyclic gas pressure changes produced by the compressor piston driven by the compressor crankshaft or linear electric motor. The gas thus supplied to and withdrawn from the expander traveled through a supply tube commonly referred to as a transfer line. The two subassemblies were thus often interconnected with a sufficiently small diameter gas transfer line to effectively decouple vibration and motion of the expander subassembly from vibration and motion of the compressor subassembly. This was particularly of interest when detectors in dewars mounted on the expander subassembly were isolated by a gimbaled mechanism from the compressor subassembly. This configuration provided a gimballing of the detector without introducing large, detrimental spring torques to the gimbal torque motors. This design permitted the compressor, which was large compared to the expander assembly, to be remotely located where available volume and heat rejection capability existed.
A natural evolution of this design was the relocation of the relatively heavy regenerator from the reciprocating displacer piston to the (stationary) cold cylinder. Thus, the displacer piston could be hollowed out making it much lighter than prior designs. This provided a reduction in the vibration output of the expander. Notwithstanding these refinements, Stirling cycle coolers continue to vibrate the associated mounting structure and, in particular, the detector assembly.
A conventional solution to this problem includes the provision of a small air gap between the cold tip of the expander and the dewar cold well to which the detector is mounted. However, this gap presents another problem: viz., how to transfer thermal energy from the detector to the cold finger of the expander. Currently two approaches are principally used in the art to interface a thermal load to a cold finger or cylinder.
One approach involves the mounting of the thermal load directly to the end of the cooler cylinder. This provides an excellent thermal interface and meets mechanical requirements. However, this approach results in an expensive integrated component with a combined cooler, sensor and dewar that cannot be easily repaired.
An alternative approach involves bonding an adapter to the cold finger with a conductive epoxy. Thermal grease is then used between the adapter and the surface to be cooled. During cooldown, thermal grease in the adapter-to-thermal load gap will freeze. As cooldown continues, differential thermal expansion of the cold finger, adapter, and dewar inner cylinder to which the thermal load is attached, causes the adapter to pull away from the dewar surface being cooled. The frozen thermal grease, acting as an adhesive, strains until it parts from the dewar surface. This may cause a spalling (removal of small fragments of material) of the metal or glass surface to be cooled. Over many cooldown cycles, debris is generated and the dewar structure of the metal or glass surface to be cooled may be compromised. In any event, the thermal interface becomes inadequate as the adapter is pulled farther from the surface to be cooled, creating a significant air gap.
Hence, a need remains in the art for an improved system or technique for interfacing the cooling surface of a cooling system to a thermal load. Particularly, there is a need in the art for a cooling surface which affords some degree of axial flexibility to compensate for differential coefficients of thermal expansion, mechanical tolerances, and other potential changes in relative location.